Dispersion Problems
Collecting surfaces of electrostatic precipitators must be periodically cleaned of collected precipitate. It is known to clean such surfaces using electromechanical devices to cause vibration and oscillation of the surfaces. These electromechanical devices included impact hammers or rappers as well as electromechanical vibrators. In electrostatic precipitators the amount of mechanical energy applied by the rappers was controlled by varying the number of full half cycles or the number of diminished conduction angle half cycles of an alternating or rectified source applied to the rapper. The amount of mechanical energy applied by the vibrators was controlled by varying the electrical power (conduction angle) applied and the duration of excitation.
In the past, either a single rap of applied mechanical energy or a plurality of raps of mechanical energy were used for each cleaning cycle. The amount of energy of the single rap on the plurality of raps was constant and was usually chosen in an attempt to dislodge all of the surface precipitate from a collecting surface. However, if excessive energy were applied, the precipitate could be sufficiently disturbed that the collected mass of the precipitate was dispersed into small particles that lacked sufficient mass density to cause them to fall free of the electrostatic precipitator. The small dispersed particles were then carried with the gas flow and were either reentrained within the electrostatic precipitator or expelled through the exhaust stack. Reentrainment of dispersed particles resulted in decreased efficiency of the electrostatic precipitator, decreased efficiency of rapper energy use and decreased control of particulate emissions.
The amount of mechanical energy received by an incremental area of the precipitate layer of a collecting surface is inversely related to the distance of the area from the point of the collecting surface where the energy is applied. Therefore precipitate areas close to the area of energy application vibrate more than areas farther from the energy application. Thus sufficient mechanical energy when applied to dislodge precipitate from more remote areas resulted in dispersion of precipitate from areas near the energy source causing possible reentrainment. If the level of applied mechanical energy was selected low enough to prevent dispersion of precipitate near the energy source, precipitate in more remote areas may not be removed.